This invention relates to radioactive well logging and more particularly to an improved method of and system for distinguishing between oil bearing zones and salt water bearing zones in subsurface formations surrounding a borehole and for further use in determining the oil saturation of an identified oil bearing zone.
In neutron-neutron logging, a steady-state source of primary radiation irradiates the formations surrounding the borehole with neutrons. The resulting secondary radiation can be measured by detectors axially spaced from such source within the borehole. The resulting secondary radiation includes epithermal neutrons, thermal neutrons, and thermal neutron capture gamma rays. For a point source in an infinite, homogeneous medium, this secondary radiation can be represented as follows: ##EQU1## WHERE, .PHI..sub.T IS THE THERMAL NEUTRON FLUX,
r is the radial distance measured from the source, PA1 Q is the point neutron source strength, PA1 D is the thermal neutron diffusion coefficient, PA1 L.sub.e is the epithermal neutron parameter (slowing down length), and PA1 L.sub.t is the thermal neutron parameter (diffusion length).
The epithermal neutron parameter L.sub.e of the formation principally is determined by the concentration of hydrogen in the formation, and hydrogen content is related to the porosity of the formation. However, the presence of oil or salt water in the pore spaces of the formation has little or not effect on the porosity. Consequently, the presence of oil or salt water in the formation has little or no effect on the number of epithermal neutrons returning to the borehole as secondary radiation.
The thermal neutron parameter L.sub.t of the formation, on the other hand, is affected by the presence of oil or salt water and is significantly reduced when the pore spaces of the formation contain salt water rather than oil. The chlorine present in the salt water has a large capture cross section for thermal neutrons and, consequently, reduces the number of thermal neutrons returning to the borehole as secondary radiation. At the same time, the capture of thermal neutrons by the chlorine effects an increase in the number of thermal neutron capture gamma rays returning to the borehole as secondary radiation.